I'm not sure how its showing for you, but the link you posted is already the thumbnail for everyone else. Also, for someone whose complaint is the UI, you sure made it seem like you thought Lemmy was a sewer, not someplace with a bad ui.
You hate the UI, except the UI is actually displaying the image on Lemmy. kbin isn't showing you the image...sounds like the UI is objectively worse over there.
Also, the UI can be changed through your client app, so Lemmy has that flexibility
Kbin is federated with several lemmy instances and it's the source of most content with community focused microblogging in the fediverse. The post you're commenting on was posted directly to LW.
Might be a bug on the platform. Maybe the pic didn't upload on your instance. I've seen that before on some very old software in the fedi. May want to talk to the kbin team/look at logs on the instance.